Predicting lamp spectrum effects at mesopic levels. Part 2: Preferred appearance and visual acuity

Laboratory tests were carried out to investigate lighting for pedestrians at mesopic levels under lamps of different spectral power distribution. This paper reports an evaluation of lamp spectrum effects on visual acuity, using Landolt ring charts of high and low luminance contrast, and forced choice judgements of the preferred appearance of human hands, a colour array and an illuminated space. These were carried out alongside judgements of brightness reported in a previous paper. Five types of lamp were used, including standard high pressure sodium, two metal halide and a fluorescent lamp of broader spectral distribution and a two-colour solid state device (LED). It was found that lamp spectrum affected judgements of preferred appearance, with the high pressure sodium and LED lamps being considered poor compared with the two metal halide lamps; these results correlated better with the CIE General Colour Rendering Index than other metrics of lamp spectral characteristics. It was also found that acuity was affected by lamp spectral power distribution, with the high pressure sodium (HPS) lamp enabling more Landolt rings to be correctly read than lamps of higher S/P ratio.